fix(stats): savings calc no longer goes negative when user spends on media

[KillerQueen-Z]

Summary

The "Saved vs Opus" hero on the panel can display a negative dollar amount as soon as a user spends meaningfully on `ImageGen` or `VideoGen`. Real example just hit:

$-8.79
You spent $20.4896 instead of $11.70

This happens because the comparison in `getStatsSummary()` mixes two different accounting universes.

Root cause

`src/stats/tracker.ts:268-272`:

```typescript
const opusCost =
(stats.totalInputTokens / 1_000_000) * OPUS_PRICING.input +
(stats.totalOutputTokens / 1_000_000) * OPUS_PRICING.output;
const saved = opusCost - stats.totalCostUsd;
```

• `opusCost` only counts chat tokens (priced at Opus rates).
• `stats.totalCostUsd` includes every recorded costUsd — chat plus image / video / music gen.
• `recordUsage` writes media calls with `inputTokens=0, outputTokens=0` because per_image / per_second / per_track billing has no token concept, so they don't enter `opusCost` but they do enter `totalCostUsd`.

`saved` therefore equals `(chat-Opus-vs-chosen-delta) − media_cost`. Once media spend exceeds the chat delta, `saved` flips negative even though every chat call was strictly cheaper than Opus would have been.

Fix

Walk `byModel` once and split spend by whether the row accumulated tokens:

• chatOnlyCost — rows that did (chat models)
• mediaCost — rows that didn't (image / video / music)

Then construct the comparison so media appears on both sides of "you spent X instead of Y":

```typescript
const opusChatCost = (totalIn / 1M) * OPUS_PRICING.input + (totalOut / 1M) * OPUS_PRICING.output;
const opusCost = opusChatCost + mediaCost; // display baseline
const totalCostUsd = chatOnlyCost + mediaCost; // unchanged
const saved = Math.max(0, opusChatCost - chatOnlyCost);
```

Media nets to zero in the diff, so `saved` correctly reflects only the chat-side win — and the `Math.max(0, ...)` clamp handles the edge case where the user deliberately picks a more expensive chat model than Opus (e.g. Sonnet 4.6 with extended thinking).

`getStatsSummary` now also returns `chatOnlyCost` and `mediaCost` for any future panel feature that wants to show the breakdown.

Worked example

	Before	After
chat spend	$2.00	$2.00
image spend	$11.30	$11.30
Opus chat baseline	$11.70	$11.70
Saved displayed	−$1.60 ❌	$9.70 ✅
"You spent"	$13.30	$13.30
"instead of"	$11.70	$23.00 (= $11.70 + $11.30)

Both displayed totals now match the user's actual wallet activity, savings is the chat-side delta only, and the number is never negative.

Files

• `src/stats/tracker.ts` — split chat/media in `getStatsSummary()`, expose both on the return type, clamp `saved` to >= 0
• `src/panel/html.ts` — defensive: clamp `saved` and `pct` to >= 0 even if a stale `stats` object lacks `saved` (older panel sessions could otherwise still hit the negative path)

```
src/panel/html.ts | 11 +++++++++--
src/stats/tracker.ts | 38 +++++++++++++++++++++++++++++++++-----
2 files changed, 42 insertions(+), 7 deletions(-)
```

Out of scope

• `src/stats/insights.ts` already has a `Math.max(0, ...)` clamp on its own savings calc, so it never displayed a negative number — but its math has the same chat/media conflation. Could be aligned in a follow-up.
• No change to `recordUsage`, the on-disk schema, or any chat-only flow.

Test plan

```bash

Reproduce the negative case (before)

franklin --prompt 'use bytedance/seedance-2.0 to generate a 5-second clip of clouds'

repeat until media spend > chat-vs-Opus delta

franklin panel # open dashboard → savings hero → see negative dollar
```

After: same flow shows "You spent $X instead of $Y, saved $Z" where Z >= 0 and X / Y both include media.