feat(web): redesign Files page filters with custom Select component

[Yevanchen]

Fill what changed. Use N/A for irrelevant or maintainer-only items. See CONTRIBUTING.md for branch, CLA, generated file, and CI rules.

Summary

• Add a shared Select component (apps/web/src/shared/ui/select.tsx) built on the Base UI Select primitive, styled with the app design tokens (card surface, border-border-strong, popover shadow, check indicator, chevron icon).
• Replace the two native <select> elements in the Files page filter toolbar (Agent filter, Thread filter) with the new component; long thread titles now truncate in both the trigger and the popup.

Why

• The Files page filters rendered as browser-native selects, which looked out of place next to the styled segmented control and search input (YEF-825 reference screenshot). The new component matches the design system and adds consistent hover/focus/open states and keyboard navigation.

Verification

• Commands: bun run fmt, bun run lint, bun run tc, bun run test (140 pass) in apps/web.
• Manual steps: ran the local dev stack, logged in via the development backdoor, and exercised the Files page — closed state, open agent dropdown, open thread dropdown (screenshots on the linked Multica issue).
• Not run: e2e suite.

Impact

• User/API/contract changes: visual/interaction change only; filter behavior and state handling are unchanged.
• Generated files / GraphQL / DB / lockfile: N/A
• Env or config changes: N/A
• Risk and rollback: low; revert the single commit to restore native selects.

Review

• Closest review areas: apps/web/src/shared/ui/select.tsx (new component), Files page toolbar wiring in apps/web/src/routes/files/files.route.tsx.
• Known trade-offs: the cost page (cost-agents-panel.tsx) still uses a native <select>; migrating it is left as a follow-up to keep this change scoped.