feat(ui): Orca Dark / Orca Light themes and notification sounds

Two optional named themes from Orca's neutral palette join the Appearance
picker (Orca Dark keeps a blue accent so white-on-accent buttons stay
legible; Orca Light's status colours are darkened to clear 4.5:1 on their
chips). A shown notification can play a sound — None (default), Chime, Pop
or Alert — chosen globally and per notification type in Settings →
Notifications; the WAVs are generated by scripts/gen-notification-sounds.js.

client/src/index.css

@@ -209,6 +209,42 @@
   --syn-plain: 229 229 234; /* #e5e5ea */
 }
 
+/* Orca Dark — Orca's neutral palette (src/renderer/src/assets/main.css `.dark`).
+ * Accent is NOT Orca's #e5e5e5 primary: AgentHub puts white text on `bg-accent`
+ * (.btn-primary), which would read 1.3:1 there; #3b82f6 keeps text-accent 4.6:1
+ * on the card. Syntax colours are Apple Dark's. */
+.dark[data-theme="orca-dark"] {
+  --surface-0: 10 10 10; /* #0a0a0a */
+  --surface-1: 17 17 17; /* #111111 */
+  --surface-2: 23 23 23; /* #171717 */
+  --surface-3: 28 28 28; /* #1c1c1c */
+  --surface-4: 38 38 38; /* #262626 */
+  --surface-5: 64 64 64; /* #404040 */
+  --border: 31 31 31; /* #1f1f1f */
+  --border-light: 46 46 46; /* #2e2e2e */
+  --accent: 59 130 246; /* #3b82f6 */
+  --accent-hover: 96 165 250; /* #60a5fa */
+  --fg-primary: 250 250 250; /* #fafafa */
+  --fg-secondary: 212 212 212; /* #d4d4d4 */
+  --fg-muted: 161 161 161; /* #a1a1a1 */
+  --status-success: 134 239 172; /* #86efac */
+  --status-danger: 255 101 104; /* #ff6568 */
+  --status-warning: 234 179 8; /* #eab308 */
+  --syn-comment: 127 140 152; /* #7f8c98 */
+  --syn-string: 252 106 93; /* #fc6a5d */
+  --syn-number: 208 191 105; /* #d0bf69 */
+  --syn-keyword: 252 95 163; /* #fc5fa3 */
+  --syn-builtin: 93 216 255; /* #5dd8ff */
+  --syn-function: 103 183 164; /* #67b7a4 */
+  --syn-operator: 161 161 166; /* #a1a1a6 */
+  --syn-punctuation: 161 161 166; /* #a1a1a6 */
+  --syn-property: 181 140 240; /* #b58cf0 */
+  --syn-tag: 252 95 163; /* #fc5fa3 */
+  --syn-add: 48 209 88; /* #30d158 */
+  --syn-del: 255 105 97; /* #ff6961 */
+  --syn-plain: 229 229 234; /* #e5e5ea */
+}
+
 [data-mode="light"][data-theme="apple-light"] {
   --surface-0: 233 233 238; /* #e9e9ee */
   --surface-1: 239 239 244; /* #efeff4 */
@@ -304,6 +340,41 @@
   --syn-del: 159 47 35; /* #9f2f23 */
 }
 
+/* Orca Light — Orca's neutral palette (`:root`). The surfaces rise with their
+ * index, so the page sits below a #fafafa card instead of Orca's white-on-white;
+ * status colours are two steps darker than Orca's so each label clears 4.5:1 on
+ * its own /20 chip. Syntax colours are Apple Light's. */
+[data-mode="light"][data-theme="orca-light"] {
+  --surface-0: 235 235 235; /* #ebebeb */
+  --surface-1: 240 240 240; /* #f0f0f0 */
+  --surface-2: 245 245 245; /* #f5f5f5 */
+  --surface-3: 250 250 250; /* #fafafa */
+  --surface-4: 253 253 253; /* #fdfdfd */
+  --surface-5: 255 255 255; /* #ffffff */
+  --border: 229 229 229; /* #e5e5e5 */
+  --border-light: 212 212 212; /* #d4d4d4 */
+  --accent: 23 23 23; /* #171717 */
+  --accent-hover: 38 38 38; /* #262626 */
+  --fg-primary: 10 10 10; /* #0a0a0a */
+  --fg-secondary: 64 64 64; /* #404040 */
+  --fg-muted: 115 115 115; /* #737373 */
+  --status-success: 22 101 52; /* #166534 */
+  --status-danger: 168 0 15; /* #a8000f */
+  --status-warning: 133 77 14; /* #854d0e */
+  --syn-comment: 93 108 121; /* #5d6c79 */
+  --syn-string: 196 26 22; /* #c41a16 */
+  --syn-number: 28 0 207; /* #1c00cf */
+  --syn-keyword: 155 35 147; /* #9b2393 */
+  --syn-builtin: 11 79 121; /* #0b4f79 */
+  --syn-function: 50 109 116; /* #326d74 */
+  --syn-operator: 75 75 80; /* #4b4b50 */
+  --syn-punctuation: 75 75 80; /* #4b4b50 */
+  --syn-property: 57 0 160; /* #3900a0 */
+  --syn-tag: 155 35 147; /* #9b2393 */
+  --syn-add: 31 122 54; /* #1f7a36 */
+  --syn-del: 197 0 15; /* #c5000f */
+}
+
 @layer base {
   * {
     scrollbar-width: thin;