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>

static/commands.js

@@ -10,7 +10,7 @@ const COMMANDS=[
   {name:'workspace', desc:'Switch workspace by name',            fn:cmdWorkspace, arg:'name'},
   {name:'new',       desc:'Start a new chat session',            fn:cmdNew},
   {name:'usage',     desc:'Toggle token usage display on/off',   fn:cmdUsage},
-  {name:'theme',     desc:'Switch theme (dark/light/solarized/monokai/nord)', fn:cmdTheme, arg:'name'},
+  {name:'theme',     desc:'Switch theme (dark/light/slate/solarized/monokai/nord)', fn:cmdTheme, arg:'name'},
 ];
 
 function parseCommand(text){
@@ -124,7 +124,7 @@ async function cmdUsage(){
 }
 
 async function cmdTheme(args){
-  const themes=['dark','slate','solarized','monokai','nord'];
+  const themes=['dark','light','slate','solarized','monokai','nord'];
   if(!args||!themes.includes(args.toLowerCase())){
     showToast('Usage: /theme '+themes.join('|'));
     return;