feat: add daily token limit with actual usage tracking (#171)

* feat: add daily token limit with actual usage tracking

- Add DAILY_TOKEN_LIMIT env var for configurable daily token limit
- Track actual tokens from Bedrock API response metadata (not estimates)
- Server sends inputTokens + cachedInputTokens + outputTokens via messageMetadata
- Client increments token count in onFinish callback with actual usage
- Add NaN guards to prevent corrupted localStorage values
- Add token limit toast notification with quota display
- Remove client-side token estimation (was blocking legitimate requests)
- Switch to js-tiktoken for client compatibility (pure JS, no WASM)

* feat: add TPM (tokens per minute) rate limiting

- Add 50k tokens/min client-side rate limit
- Track tokens per minute with automatic minute rollover
- Check TPM limit after daily limits pass
- Show toast when rate limit reached
- NaN guards for localStorage values

* feat: make TPM limit configurable via TPM_LIMIT env var

* chore: restore cache debug logs

* fix: prevent race condition in TPM tracking

checkTPMLimit was resetting TPM count to 0 when checking, which
overwrote the count saved by incrementTPMCount. Now checkTPMLimit
only reads and incrementTPMCount handles all writes.

* chore: improve TPM limit error message clarity

package-lock.json

@@ -37,6 +37,7 @@
                 "base-64": "^1.0.0",
                 "class-variance-authority": "^0.7.1",
                 "clsx": "^2.1.1",
+                "js-tiktoken": "^1.0.21",
                 "jsdom": "^26.0.0",
                 "lucide-react": "^0.483.0",
                 "next": "^16.0.7",
@@ -6290,6 +6291,26 @@
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             "license": "MIT"
         },
+        "node_modules/base64-js": {
+            "version": "1.5.1",
+            "resolved": "https://registry.npmjs.org/base64-js/-/base64-js-1.5.1.tgz",
+            "integrity": "sha512-AKpaYlHn8t4SVbOHCy+b5+KKgvR4vrsD8vbvrbiQJps7fKDTkjkDry6ji0rUJjC0kzbNePLwzxq8iypo41qeWA==",
+            "funding": [
+                {
+                    "type": "github",
+                    "url": "https://github.com/sponsors/feross"
+                },
+                {
+                    "type": "patreon",
+                    "url": "https://www.patreon.com/feross"
+                },
+                {
+                    "type": "consulting",
+                    "url": "https://feross.org/support"
+                }
+            ],
+            "license": "MIT"
+        },
         "node_modules/baseline-browser-mapping": {
             "version": "2.8.31",
             "resolved": "https://registry.npmjs.org/baseline-browser-mapping/-/baseline-browser-mapping-2.8.31.tgz",
@@ -8851,6 +8872,15 @@
                 "jiti": "lib/jiti-cli.mjs"
             }
         },
+        "node_modules/js-tiktoken": {
+            "version": "1.0.21",
+            "resolved": "https://registry.npmjs.org/js-tiktoken/-/js-tiktoken-1.0.21.tgz",
+            "integrity": "sha512-biOj/6M5qdgx5TKjDnFT1ymSpM5tbd3ylwDtrQvFQSu0Z7bBYko2dF+W/aUkXUPuk6IVpRxk/3Q2sHOzGlS36g==",
+            "license": "MIT",
+            "dependencies": {
+                "base64-js": "^1.5.1"
+            }
+        },
         "node_modules/js-tokens": {
             "version": "4.0.0",
             "resolved": "https://registry.npmjs.org/js-tokens/-/js-tokens-4.0.0.tgz",