fix: repo dropdown can misalign after window resize

[michaelzwang13]

Problem

The custom repo dropdown on the Signal Feed (RepoSelect in app/src/pages/Agents.tsx) is anchored via position: absolute + left: 0 relative to its trigger, but the trigger's position changes when the filter row wraps at narrower viewports. If the user opens the menu and then resizes the window — or if the menu's position was computed against a no-longer-current trigger location — the menu can land off-screen or visually drift away from its trigger.

The earlier fix (#38) addressed the static narrow-viewport case by switching from right: 0 to left: 0 + width: max-content + maxWidth: 280. That handles the initial render at any width, but not dynamic resize while the dropdown is open.

Repro

1. Open /agents at a wide viewport (~1440px)
2. Click the repo dropdown so the menu is open
3. Drag the window narrower (~900px or below) so the filter row wraps
4. Menu now sits at the old trigger position, may be partially off-screen or detached from its trigger

Approach (sketch)

Three options, in increasing complexity:

1. Close the dropdown on window.resize — simplest, slightly annoying UX
2. Force re-render on resize so the absolute-positioned menu re-anchors — listen for resize, force a state bump while the menu is open
3. Use a positioning library (e.g. @floating-ui/react) to compute placement dynamically with collision detection — more weight, but the right shape if more menus get added

Recommend (2) for now; revisit (3) if menu count grows.

Files

• app/src/pages/Agents.tsx — RepoSelect component (the click-outside / Escape handlers are already on a useEffect gated by open; add resize to the same listener block)

Acceptance

• Resizing the window with the menu open keeps the menu anchored to its trigger (or closes the menu cleanly)
• No regression in the narrow-viewport static case fixed in feat: richer GitHub Signal Feed + caching layer #38
• Manual check at 1440px, 1024px, 768px

Out of scope

• Replacing the custom dropdown with a portal/floating-ui-based component (separate refactor)