refactor(i18n) : replace hardcoded French strings with i18n keys

Replace 30+ hardcoded strings across 15 components with $t() calls.
Added keys for common actions, drawers titles, empty states, and modals.

Ticket: T-020

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

frontend/components/time-tracking/TimeTrackingCalendar.vue

@@ -99,7 +99,7 @@
                             :style="{ backgroundColor: entry.project?.color ?? '#94a3b8' }"
                         />
                         <div class="min-w-0">
-                            <div class="truncate text-xs font-medium text-neutral-800">{{ entry.title || 'Sans titre' }}</div>
+                            <div class="truncate text-xs font-medium text-neutral-800">{{ entry.title || $t('common.untitled') }}</div>
                             <div class="text-[10px] text-neutral-500">
                                 {{ formatTime(entry.startedAt) }} – {{ entry.stoppedAt ? formatTime(entry.stoppedAt) : '...' }}
                             </div>
@@ -141,6 +141,8 @@
 <script setup lang="ts">
 import type { TimeEntry } from '~/services/dto/time-entry'
 
+const { t } = useI18n()
+
 const props = defineProps<{
     entries: TimeEntry[]
     startDate: Date
@@ -459,7 +461,7 @@ function onMoveStart(payload: { entry: TimeEntry; offsetY: number }, sourceDayIn
     dragState.value = {
         entryId: entry.id,
         entry,
-        title: entry.title || 'Sans titre',
+        title: entry.title || t('common.untitled'),
         color: entry.project?.color ?? '#94a3b8',
         durationMinutes,
         ghostHeightPx: Math.max((durationMinutes / 60) * hourHeight, 20),