feat: massive overhaul to manage costs (#103)

### Explanation

_No response_

### Issue

Closes #102

### Attestations

- [ ] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
- [ ] I have read and agree to the [Community Guidelines](https://docs.nhcarrigan.com/community/guide/).
- [ ] My contribution complies with the [Contributor Covenant](https://docs.nhcarrigan.com/dev/covenant/).

### Dependencies

- [ ] I have pinned the dependencies to a specific patch version.

### Style

- [ ] I have run the linter and resolved any errors.
- [ ] My pull request uses an appropriate title, matching the conventional commit standards.
- [ ] My scope of feat/fix/chore/etc. correctly matches the nature of changes in my pull request.

### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
- [ ] My contribution modifies existing code, and I have updated the tests to reflect these changes.
- [ ] All new and existing tests pass locally with my changes.
- [ ] Code coverage remains at or above the configured threshold.

### Documentation

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### Versioning

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Reviewed-on: #103
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src/lib/components/Terminal.svelte

@@ -211,6 +211,16 @@
       {#each lines as line (line.id)}
         <div class="terminal-line mb-2 {getLineClass(line.type)} relative group">
           <span class="terminal-timestamp text-xs mr-2">{formatTime(line.timestamp)}</span>
+          {#if line.cost && line.cost.costUsd > 0}
+            <span
+              class="terminal-cost text-xs mr-2"
+              title="Input: {line.cost.inputTokens} | Output: {line.cost.outputTokens}"
+            >
+              ${line.cost.costUsd < 0.01
+                ? line.cost.costUsd.toFixed(4)
+                : line.cost.costUsd.toFixed(3)}
+            </span>
+          {/if}
           {#if getLinePrefix(line.type)}
             <span class="terminal-prefix mr-2">{getLinePrefix(line.type)}</span>
           {/if}
@@ -291,6 +301,14 @@
     color: var(--text-tertiary, #6b7280);
   }
 
+  .terminal-cost {
+    color: var(--terminal-cost, #10b981);
+    background: var(--terminal-cost-bg, rgba(16, 185, 129, 0.1));
+    padding: 0 4px;
+    border-radius: 3px;
+    font-family: monospace;
+  }
+
   .terminal-prefix {
     color: var(--text-secondary);
   }