[players] Migrate annotation stack to fabric.js 7

[frankrousseau]

Problems

• Kitsu was pinned to fabric@6.9.1 while fabric 7 is the current major (canvas 3, node 20+, jsdom 26).
• fabric 7 flips the default originX/originY to 'center': every Kitsu shape is created and serialised by its top-left corner, so new drawings and previously-saved annotations would render shifted by half a dimension.
• fabricjs-psbrush was still pinned to a personal fork branch instead of the upstream cgwire repo.
• Clicking an annotation marker landed on the previous frame about half the time (float floor vs the round used to draw the marker).
• Shift-scrub did not drive the comparison player in the playlist view.
• Drawing while navigating frames quickly could lose annotations: the canvas reloads asynchronously without cancellation, so a save could replace an annotation with a partially-loaded canvas, and legacy unrounded annotation times left duplicated same-frame entries of which only one ever displays.

Solutions

• Bump fabric to 7.4.0 and repoint fabricjs-psbrush to cgwire/fabricjs-psbrush (peer range widened to ^6 || ^7 in [deps] Accept fabric 7 as a peer dependency fabricjs-psbrush#2, prerequisite).
• Restore the v6 left/top default origin globally via FabricObject.ownDefaults at annotation module init, with a regression test.
• Compute the clicked annotation frame with Math.round, matching the marker position; bar clicks keep floor.
• Sync the comparison player from the shift-scrub handler, like every other navigation path.
• Cancel in-flight annotation loads on canvas clear (load token), merge canvas objects by id instead of replacing the annotation drawing on save, and merge duplicated same-frame entries at load time (mergeAnnotationsByFrame). Server-side cleanup of the legacy entries: [previews] Add a command to normalize annotation times zou#1105.
• Gates: full unit suite (909 tests) and prod build green; runtime checklist validated by hand (draw + reload of every tool, pre-v7 annotation data, eraser mask, fullscreen/resize, playlist comparison, right-click).

[frankrousseau]

Follow-up (non-blocking): the cgwire/fabric.js v5-snapshot fork is no longer referenced by Kitsu since the v6 migration — it can be archived once we confirm nothing else depends on it.

[frankrousseau]

The annotation-loss fix in this branch has a server-side complement: cgwire/zou#1105 adds a zou normalize-annotation-times command that merges the legacy duplicated entries in base and snaps their times onto the player's frame grid. Until it is run, deleting a legacy-timed object can make it reappear on reload (the deletion targets the canonical time, not the stored legacy one).