docs: v0.34 release — themes CHANGELOG, README, add light to picker

- CHANGELOG: v0.34 Sprint 26 entry (6 themes, /theme command, settings UX)
- README: themes section, updated slash commands, THEMES.md in docs list
- THEMES.md: added Slate to theme table, matches actual CSS/dropdown
- commands.js: added 'light' to /theme valid list, updated description
- index.html: added Light option to theme dropdown, version v0.34
- SPRINTS/CHANGELOG footers updated to v0.34 / 433 tests

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

SPRINTS.md

@@ -1,6 +1,6 @@
 # Hermes Web UI -- Forward Sprint Plan
 
-> Current state: v0.32 | 424 tests | Daily driver ready
+> Current state: v0.34 | 433 tests | Daily driver ready
 > This document plans the path from here to two targets:
 >
 > Target A: 1:1 feature parity with the Hermes CLI (everything you can do from the
@@ -1156,6 +1156,6 @@ New test cases in `tests/test_sprint26.py`:
 ---
 
 *Last updated: April 5, 2026*
-*Current version: v0.33 | 424 tests*
+*Current version: v0.34 | 433 tests*
 *Next sprint: Sprint 24 (Web Polish + Bug Fix Pass)*
 *Horizon sprint: Sprint 26 (Pluggable UI Themes)*