[Feature Request] Credit/Billing Transparency: Real-time consumption visibility and budget controls

## Is your feature request related to a problem?

As a heavy user on the personal premium plan (99K credits/month), I find it extremely difficult to manage credit consumption. There is no way to:

1. **See real-time credit balance** during a conversation — you only discover you've run out when operations start failing silently
2. **Get per-operation cost estimates** before executing expensive operations (image generation, browser automation, PPT creation)
3. **Set budget alerts or limits** — no warning when approaching the monthly cap
4. **View detailed consumption breakdown** — the dashboard shows total usage but not which operations consumed what

This leads to:
- Unexpected mid-task failures when credits run out
- Sub-agents continuing to execute after credits are depleted, producing garbage results
- No way to optimize spending patterns because you can't see them

## Describe the solution you'd like

### 1. Real-time credit indicator in the chat UI
- Show remaining credits prominently (like a battery indicator)
- Color-coded: green (>50%), yellow (20-50%), red (<20%)
- Update after each significant operation

### 2. Per-operation cost preview
- Before executing high-cost operations (image gen, video gen, browser use, large file parsing), show estimated credit cost
- Allow users to confirm or cancel before proceeding
- Configurable threshold (e.g., warn for operations >100 credits)

### 3. Budget controls
- Set daily/monthly soft limits with warnings
- Set hard limits that pause non-essential operations
- Sub-agent budget inheritance — parent can set per-sub-agent limits

### 4. Consumption analytics dashboard
- Breakdown by operation type (chat, search, file processing, media generation, browser)
- Daily/weekly/monthly trends
- Export as CSV for external analysis

## Describe alternatives you've considered

- **Manual credit checking via API**: I built a bash adapter (`credit_check.js`) that queries the balance, but this is a workaround, not a solution. Most users won't do this.
- **Spreadsheets to track usage**: Manual and error-prone
- **Avoiding expensive operations entirely**: This defeats the purpose of having a powerful platform

## Additional context

This is the single most impactful improvement for power users. Credit anxiety is the #1 friction point I've experienced and heard from other heavy users. The platform has powerful capabilities but users are afraid to use them because they can't see the cost.

I've documented detailed consumption patterns across different operation types and model configurations if the team wants reference data.

**Priority**: P0 — Directly impacts user trust and platform adoption for paid tiers.

No files, tests, or entry points are named. Start by breaking the request into a scoped milestone—credit indicator, cost previews, budget controls, or analytics—and identify the existing balance and usage surfaces; done should be defined by the selected milestone’s UI behavior and supporting enforcement or reporting tests.

Written by the indexing model from the issue text.