fix: Delete/Backspace removes the selected timeline clip

[thisiscsim]

Summary

• Clicking a clip and pressing Delete (or Backspace) now removes it. Previously no delete shortcut existed — the only removal affordance was the empty-layer trash button in the track gutter, which agent-generated tracks (always populated) never show, making their clips look undeletable.
• New removeClip helper searches every layer by clip id (video/text/audio; caption layers have no clips), so agent-generated and hand-placed clips delete identically — no id-pattern assumptions.
• The shortcut lives with the other editor-level keys in App.tsx: skips text fields/contentEditable so typing is unaffected, reads selection through getState() at keypress time, clears the selection after, and routes through updateEdl so deletion participates in undo (Cmd+Z brings the clip back) and autosave.

Test plan

• npm run typecheck && npm test && npm run build green (152 tests, 2 new for removeClip)
• Select an agent-generated video clip → Delete removes it; Cmd+Z restores it
• Same for text and audio clips, and for Backspace
• Typing in the composer / rename fields: Backspace edits text, never deletes clips

Renderer-only change — HMR covers it, no dev restart needed.

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