Vertical hover overlay follow-up + remove sidebar tab-search button

[dusan-nikcevic]

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Tested on (check one or more):

• Windows
• macOS
• Linux

---

Follow-up to #964 for approved issue #915 (vertical tabs hover expand):

• Keep collapsed vertical tabs reserved width stable while drawing hover expansion as overlay.
• Wire hover enter/exit and hover-expand state through vertical tab strip layout callers.
• Add smooth hover expand/collapse animation path (avoid jumpy size changes).
• Add command + settings pref plumbing for collapse/hover behavior.
• Remove persistent vertical tab-search sidebar button from layout (no top-left search icon in vertical sidebar).
• Add two upstream-compat build patches that unbreak current tree compilation:
  • autofill-async-context-model-predictions.patch
  • passage-embeddings-forward-decl.patch

Local validation:

• python3 ./devutils/lint.py passes.
• ninja -C out/DevFast chrome succeeds in local Chromium worktree.
• Manual runtime verification on Linux with DevFast build and vertical tabs enabled.

Example:

2026-02-27.00-12-54.online-video-cutter.com.mp4

[quanticstudios]

Moved this work to from quanticstudios fork. Keeping this open briefly for reviewer continuity.

[quanticstudios]

Correct link: moved this work to #1014 from quanticstudios fork. Keeping this PR open briefly for reviewer continuity.