feat: Sprint 16 — session sidebar visual polish

- Action buttons overlay: wrap pin/move/archive/dup/trash in a
  .session-actions container with position:absolute. Titles now use
  full available width. Actions appear on hover with gradient fade
  from the right edge. Overlay auto-hides during inline rename.

- SVG line icons: replace all emoji HTML entities with monochrome
  SVGs that inherit currentColor. Consistent across all platforms.

- Pin indicator: small gold star rendered inline only when pinned.
  Unpinned sessions get full title width (zero space reservation).

- Project border: sessions assigned to a project show a colored
  left border matching the project color, replacing the old
  always-visible blue folder button.

Fixes both BUGS.md items (title truncation + sticky folder button).
Tests: 214 passed, 23 pre-existing failures, 0 regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

BUGS.md

@@ -0,0 +1,18 @@
+# Bugs Backlog
+
+This file tracks UI bugs and polish items to address in a future sprint.
+
+## ~~Conversation list title truncation / hover actions~~ — Fixed (Sprint 16)
+
+- **Was:** Action icons reserved ~30px of space even when invisible, truncating titles.
+- **Fix:** Wrapped all action buttons in a `.session-actions` overlay container with `position:absolute`. Titles now use full available width. Actions appear on hover with a gradient fade from the right edge.
+
+## ~~Folder/project assignment interaction feels sticky~~ — Fixed (Sprint 16)
+
+- **Was:** Folder icon stayed permanently visible (blue, 60% opacity) when a session belonged to a project.
+- **Fix:** Replaced `.has-project` persistent button with a colored left border matching the project color. The folder button now only appears in the hover overlay like all other actions.
+
+## Notes
+
+- Both issues resolved in Sprint 16 (Session Sidebar Visual Polish).
+- Icons replaced from inconsistent emoji HTML entities to monochrome SVG line icons.