fix: replace invalid FFmpeg fontweight param with bold=1 and fix fontfile set to name instead of path

[abhinav-atul]

Description

Fixes two bugs in buildTextFilter() in src/lib/text-overlay.ts that silently broke bold text rendering and custom font loading in all video exports.

Bug A — Invalid fontweight= parameter:
fontweight= is not a valid FFmpeg drawtext option. FFmpeg silently ignores unknown parameters, so bold text was never rendered bold in any export. Replaced :fontweight=${fontWeightParam} with :bold=1, appended only when fontWeightParam === "bold". Omitted entirely for normal weight.

Bug B — fontfile= set to font name instead of file path:
For any overlay with a fontFamily, the filter incorrectly appended fontfile='Arial' (a sanitized font name) instead of a filesystem path. FFmpeg expects a path here, not a name, causing it to silently fail. For custom fonts, getFFmpegFontArg then appended the real fontfile= path immediately after, producing a duplicate fontfile= key. Removed the block that set fontfile= to a sanitized name. System fonts now use font='FontFamily' (the correct FFmpeg option). Custom font paths via fontFileParam are appended only when present.

Before (broken):

drawtext=...:fontweight=bold:fontfile='Arial'

After (correct):

drawtext=...:bold=1:font='Arial'                  # system font, bold
drawtext=...:bold=1:fontfile=/tmp/MyFont.ttf       # custom font, bold
drawtext=...:font='Arial'                          # system font, normal weight

Related Issue

Closes #1414

Type of Contribution

• Bug fix
• New feature
• Documentation update
• Refactor
• GSSoC contribution

Participant Info

• GitHub username: abhinav-atul
• Contribution level (Beginner/Intermediate/Advanced): Intermediate

Screen Recording

This is a non-UI logic-only change in the FFmpeg filter string builder (src/lib/text-overlay.ts). No visual UI elements were added or modified. The fix corrects the generated FFmpeg command parameters which are only visible during video export processing.

Recording / Loom link: N/A (non-UI backend logic fix — no visual changes to record)

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• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (required for UI/feature/design changes)

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[github-actions]

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[abhinav-atul]

@magic-peach Hi! The auto-label bot assigned level:beginner to this PR based on the diff size (17 lines), but I'd like to request a level:intermediate relabel. While the diff is small, this fix required:

1. Understanding FFmpeg's drawtext filter API to identify that fontweight= is not a valid parameter and the correct replacement is bold=1
2. Diagnosing a subtle bug where fontfile= was set to a font name instead of a filesystem path, causing silent failures
3. Handling the distinction between system fonts (font=) vs custom uploaded fonts (fontfile=) to avoid duplicate keys in the filter string

The issue itself is also labeled as bug and type:bug, not good first issue. Could you consider relabeling this to level:intermediate? Thank you!