feat(tui): add mouse-drag sidebar resize

[eggfriedrice24]

Issue for this PR

Closes #6087

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Makes the TUI right sidebar resizable by dragging its left edge. The sidebar width was previously hard-coded to 42 columns in sidebar.tsx and referenced again in routes/session/index.tsx; there was no way to change it short of patching the binary.

Behavior:

• 1-col resize handle sits at the sidebar/chat boundary. Idle: invisible (same bg as the panel). Hover/drag: highlighted in theme.warning (same yellow used by the permission prompt triangle, for visual consistency).
• Mouse drag adjusts width live; the persisted value is written once on drag-end, not on every pixel, to avoid thrashing the kv.json disk writes.
• Width clamped to [20, min(80, floor(terminal_width / 2))] - the dynamic upper bound prevents the sidebar from swallowing the chat on borderline-wide terminals (~120 cols).
• Persisted via the existing kv.signal under sidebar_width, survives restarts.
• Resize UI only appears in wide (inline) mode - in narrow/overlay mode the sidebar floats over the chat, where resizing has no meaning.

This picks up the design from #5917 (mouse-drag, width-persist, 20–80 clamp, 1px handle) which @agustif got a thumbs-up for from the team before it auto-closed at 60 days stale. The codebase had drifted too much (kv.signal refactor, no-floating-promises rule, etc.) for a clean rebase, so this is a fresh implementation with the same UX. @agustif is credited via Co-Authored-By on the commit.

Previous attempts:

• feat: make sidebar resizable #5504 - keybind-based approach; maintainer (@rekram1-node) suggested mouse instead
• feat: add draggable sidebar resize functionality #5917 - mouse approach that got team approval but went stale
• feat(tui): add split sidebars with independent resize controls + team setup guide #14271 - unrelated split-sidebar work, auto-closed for no linked issue

Opening as draft per CONTRIBUTING.md's design-approval expectation on UI features - happy to flip to ready-for-review once approved, or close if you'd prefer to discuss the design more on #6087 first.

How did you verify your code works?

• bun run typecheck passes clean (tsgo, cache-bypassed full run)
• bunx oxlint on the changed files: 0 new warnings (19 baseline → 19 after)
• Ran locally with bun dev . - hovering the sidebar/chat boundary highlights the handle yellow, drag resizes, value persists across restarts. Tested at 120/160/200 terminal widths; dynamic clamp keeps chat readable.

Screenshots / recordings

21.16.49_21-04-2026.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[agustif]

ty for picking this back up hope this time gets merged!

[liao666brant]

This feature seems useful, how is it going now?

[eggfriedrice24]

Your design did the heavy lifting, appreciate the original work and the doc trail you left on #5917. Now we wait on maintainer review; happy to address anything they want changed.

[eggfriedrice24]

Draft, waiting on maintainer design approval per CONTRIBUTING.md. Code is ready and verified locally, nothing pending on my side.

[notchrisbutler]

@thdxr Hi Dax! Any chance this could get a peek? Would love this..