feat(ui): add init repository dialog

Add a modal dialog to initialize a new Git repository, replacing the
previous prompt-based flow. It integrates with the app state and invokes
the initialization operation then opens the repository on confirmation.
Also adds styles, keyboard handling, accessibility hints, a folder
browser, initial-branch input, and German/English labels with busy state.

- New dialog component with folder picker, branch input, and i18n.
- Wire dialog open/close and confirm handler to init and open actions.
- Add CSS rules for dialog layout, controls, and error/busy states.

src/app.css

@@ -3878,6 +3878,13 @@
     max-height: calc(100vh - 32px);
     overflow: auto;
   }
+  .init-repository-dialog {
+    display: block;
+    width: min(540px, calc(100vw - 32px));
+    height: auto;
+    max-height: calc(100vh - 32px);
+    overflow: auto;
+  }
   .rename-branch-dialog {
     display: block;
     width: min(520px, calc(100vw - 32px));
@@ -4172,6 +4179,34 @@
     gap: 14px;
     padding: 16px;
   }
+  .init-repository-header { padding: 16px 18px; }
+  .init-repository-heading { display: flex; align-items: center; gap: 12px; }
+  .init-repository-heading h2 { margin: 2px 0 0; color: var(--color-ink); font-size: 15px; line-height: 1.25; }
+  .init-repository-icon {
+    display: grid;
+    place-items: center;
+    width: 36px;
+    height: 36px;
+    flex: 0 0 auto;
+    border: 1px solid color-mix(in srgb, var(--color-accent) 34%, var(--color-border));
+    border-radius: 9px;
+    color: var(--color-accent);
+    background: color-mix(in srgb, var(--color-accent) 10%, var(--color-surface-raised));
+  }
+  .init-repository-form { display: flex; flex-direction: column; gap: 16px; padding: 18px; }
+  .init-repository-description { margin: 0; color: var(--color-ink-dim); font-size: 11.5px; line-height: 1.5; }
+  .init-repository-field > span { font-size: 9.5px; }
+  .init-repository-field > div { position: relative; align-items: center; }
+  .init-repository-field > div > svg { position: absolute; left: 11px; z-index: 1; color: var(--color-ink-faint); pointer-events: none; }
+  .init-repository-field > div > input { height: 36px; padding-left: 36px; font-family: var(--font-mono); font-size: 12px; }
+  .init-repository-field small { color: var(--color-ink-faint); font-size: 9.5px; line-height: 1.4; }
+  .init-repository-path-field { padding: 12px; border: 1px solid var(--color-border-subtle); border-radius: 9px; background: var(--color-surface-raised); }
+  .init-repository-path-control { display: grid !important; grid-template-columns: minmax(0, 1fr) auto; gap: 8px; }
+  .init-repository-path-control > input { grid-column: 1; }
+  .init-repository-browse { grid-column: 2; display: inline-flex; align-items: center; gap: 6px; height: 36px; font-size: 11.5px; white-space: nowrap; }
+  .init-repository-error { color: var(--color-danger, #ff6b78) !important; }
+  .init-repository-actions { padding-top: 2px; }
+  .init-repository-actions > button { font-size: 11.5px; }
   .rename-branch-form {
     display: flex;
     flex-direction: column;