feat(stats): "custom only" checkbox on the symbols bubble chart

[talpitoo]

What

Adds a "custom only" checkbox to the symbols bubble chart on the Stats page — title left, checkbox right-aligned on the same line (the sleep chart's "smooth" header pattern). Checking it hides the opted-in built-in symbols so the user's own creations get the room, matching the Symbols page's checkbox semantics.

How

• No new DB query, no refetch: getDreams already ships each dream's full symbol rows to the client and setChartsData aggregates there — the entries now carry each symbol's builtIn flag, and StatSymbolChart filters on it locally ("custom" = !builtIn; a symbol attached to your own dream is either an opted-in built-in or your own creation).
• Checkbox state persists in sessionStorage like the other stats toggles (new key in range.ts).
• Fully local to StatSymbolChart: the Filters panel, range buttons, and the other facet charts are untouched; the advanced-charting view gets the checkbox automatically since it renders the same component; the PDF export (isPdf) keeps rendering the full set with no checkbox.

Testing

• New unit test (written first, watched fail) for the builtIn passthrough in chartsData.test.ts; existing by-id test loosened to objectContaining.
• npm test (129 passing), npm run type:check, eslint all green.
• Verified empirically with puppeteer as the seeded zhuangzi user: checkbox sits on the title line, toggling live-filters 26 bubbles → the 3 custom symbols (rendered much larger), state survives a reload, no new console errors.

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