feat(version): version x.y.z propre et cohérente partout + bump script

Corrige l'incoherence d'affichage (page principale 0.0.0-dev vs a propos
v2.0.0-dev) et unifie la gestion de version sur une seule source de verite.

Backend:
- version.py: get_version() retourne toujours le tag x.y.z propre du dernier
  tag (plus de 0.0.0-dev / -N-gHASH dans l'UI). Ajout get_git_describe() et
  get_git_commit() pour le detail.
- main.py: /api/health expose git_describe + git_commit (nouveaux champs).

Frontend:
- modale A propos (populateAbout): version + commit lus depuis /api/health,
  suppression du hardcode v2.0.0-dev dans index.html.
- section config A propos (loadAbout): lit health.version, plus de state.
  APP_VERSION obsolete.
- badge header: fetch /api/health (fallback neutre '...').
- state.js: suppression de APP_VERSION (1.5.0) devenu mort; imports nettoyes.

Script:
- scripts/bump_version.sh: incrementation SemVer selon les commits
  (breaking->major, feat->minor, sinon patch), creer/pousser le tag vX.Y.Z.

Tests: 507 passed, frontend validators OK.

tests/frontend/unit.test.mjs

@@ -28,7 +28,7 @@ function testEscapeHtml() {
 // ── Test state object structure ────────────────────────────────────────────
 // Replicate state.js structure for testing
 const expectedStateKeys = [
-  'APP_VERSION', 'currentVault', 'currentPath', 'allVaults', 'selectedContextVault',
+  'currentVault', 'currentPath', 'allVaults', 'selectedContextVault',
   'searchTimeout', 'searchAbortController', 'advancedSearchOffset', 'advancedSearchTotal',
   'advancedSearchSort', 'advancedSearchLastQuery', 'suggestAbortController',
   'dropdownActiveIndex', 'dropdownItems', 'currentSearchId', 'selectedTags',
@@ -43,7 +43,7 @@ const expectedStateKeys = [
 
 function testStateKeys() {
   const state = {
-    APP_VERSION: "1.5.0", currentVault: null, currentPath: null, allVaults: [],
+    currentVault: null, currentPath: null, allVaults: [],
     selectedContextVault: "all", searchTimeout: null, searchAbortController: null,
     advancedSearchOffset: 0, advancedSearchTotal: 0, advancedSearchSort: "relevance",
     advancedSearchLastQuery: "", suggestAbortController: null, dropdownActiveIndex: -1,