smart context v3 (#1022)

<!-- This is an auto-generated description by cubic. -->

## Summary by cubic
Adds Smart Context v3 with selectable modes (Off, Conservative,
Balanced) and surfaces token savings in chat. Also improves token
estimation by counting per-file tokens when Smart Context is enabled.

- **New Features**
- Smart Context selector in Pro settings with three options.
Conservative is the default when enabled without an explicit choice.
- New setting: proSmartContextOption ("balanced"); undefined implies
Conservative.
- Engine now receives enable_smart_files_context and smart_context_mode.
- Chat shows a DyadTokenSavings card when the message contains
token-savings?original-tokens=...&smart-context-tokens=..., with percent
saved and a tooltip for exact tokens.
- Token estimation uses extracted file contents for accuracy when Pro +
Smart Context is on; otherwise falls back to formatted codebase output.

<!-- End of auto-generated description by cubic. -->

e2e-tests/snapshots/engine.spec.ts_send-message-to-engine---smart-context-balanced-1.aria.yml

@@ -0,0 +1,6 @@
+- paragraph: "[dump] tc=turbo-edits"
+- paragraph: "[[dyad-dump-path=*]]"
+- img
+- text: less than a minute ago
+- button "Retry":
+  - img

e2e-tests/snapshots/smart_context_options.spec.ts_switching-smart-context-mode-saves-the-right-setting-1.txt

@@ -0,0 +1,28 @@
+{
+  "selectedModel": {
+    "name": "auto",
+    "provider": "auto"
+  },
+  "providerSettings": {
+    "auto": {
+      "apiKey": {
+        "value": "testdyadkey",
+        "encryptionType": "plaintext"
+      }
+    }
+  },
+  "telemetryConsent": "unset",
+  "telemetryUserId": "[UUID]",
+  "hasRunBefore": true,
+  "enableDyadPro": true,
+  "experiments": {},
+  "lastShownReleaseNotesVersion": "[scrubbed]",
+  "enableProLazyEditsMode": true,
+  "enableProSmartFilesContextMode": true,
+  "selectedTemplateId": "react",
+  "selectedChatMode": "build",
+  "enableAutoFixProblems": false,
+  "enableAutoUpdate": true,
+  "releaseChannel": "stable",
+  "isTestMode": true
+}

e2e-tests/snapshots/smart_context_options.spec.ts_switching-smart-context-mode-saves-the-right-setting-2.txt

@@ -0,0 +1,29 @@
+{
+  "selectedModel": {
+    "name": "auto",
+    "provider": "auto"
+  },
+  "providerSettings": {
+    "auto": {
+      "apiKey": {
+        "value": "testdyadkey",
+        "encryptionType": "plaintext"
+      }
+    }
+  },
+  "telemetryConsent": "unset",
+  "telemetryUserId": "[UUID]",
+  "hasRunBefore": true,
+  "enableDyadPro": true,
+  "experiments": {},
+  "lastShownReleaseNotesVersion": "[scrubbed]",
+  "enableProLazyEditsMode": true,
+  "enableProSmartFilesContextMode": true,
+  "proSmartContextOption": "balanced",
+  "selectedTemplateId": "react",
+  "selectedChatMode": "build",
+  "enableAutoFixProblems": false,
+  "enableAutoUpdate": true,
+  "releaseChannel": "stable",
+  "isTestMode": true
+}

e2e-tests/snapshots/smart_context_options.spec.ts_switching-smart-context-mode-saves-the-right-setting-3.txt

@@ -0,0 +1,28 @@
+{
+  "selectedModel": {
+    "name": "auto",
+    "provider": "auto"
+  },
+  "providerSettings": {
+    "auto": {
+      "apiKey": {
+        "value": "testdyadkey",
+        "encryptionType": "plaintext"
+      }
+    }
+  },
+  "telemetryConsent": "unset",
+  "telemetryUserId": "[UUID]",
+  "hasRunBefore": true,
+  "enableDyadPro": true,
+  "experiments": {},
+  "lastShownReleaseNotesVersion": "[scrubbed]",
+  "enableProLazyEditsMode": true,
+  "enableProSmartFilesContextMode": false,
+  "selectedTemplateId": "react",
+  "selectedChatMode": "build",
+  "enableAutoFixProblems": false,
+  "enableAutoUpdate": true,
+  "releaseChannel": "stable",
+  "isTestMode": true
+}

e2e-tests/snapshots/smart_context_options.spec.ts_switching-smart-context-mode-saves-the-right-setting-4.txt

@@ -0,0 +1,28 @@
+{
+  "selectedModel": {
+    "name": "auto",
+    "provider": "auto"
+  },
+  "providerSettings": {
+    "auto": {
+      "apiKey": {
+        "value": "testdyadkey",
+        "encryptionType": "plaintext"
+      }
+    }
+  },
+  "telemetryConsent": "unset",
+  "telemetryUserId": "[UUID]",
+  "hasRunBefore": true,
+  "enableDyadPro": true,
+  "experiments": {},
+  "lastShownReleaseNotesVersion": "[scrubbed]",
+  "enableProLazyEditsMode": true,
+  "enableProSmartFilesContextMode": true,
+  "selectedTemplateId": "react",
+  "selectedChatMode": "build",
+  "enableAutoFixProblems": false,
+  "enableAutoUpdate": true,
+  "releaseChannel": "stable",
+  "isTestMode": true
+}