Current behavior

`app/src/main/files-handler.ts` rejects `files:read` for any file > 5 MB (`MAX_READ_BYTES`) with a flat error. `app/src/renderer/files/file-viewer.ts` never gets bytes for those files, so the File tab shows nothing useful — testers hit this on real bioinformatics outputs (FASTQ, large VCF, BAM-derived TSVs).

Proposal

For text files past the limit, stream just a head preview instead of rejecting:

1. First 10 lines, capped on bytes — if those 10 lines together exceed e.g. 64 KB (a single uncompressed VCF data line can be tens of KB; rare but happens), truncate the rendered text and append a marker.
2. Render in the File tab with a banner: "Showing first 10 lines of ({size} total). Open externally to see the full file."
3. Hide the editor / save controls — preview-only.

Decisions to make:

• Where to draw the line. Today: hard reject at 5 MB. Proposal: full read up to 5 MB, head-preview between 5 MB and (say) 1 GB, hard reject above 1 GB.
• Whether to extend the head-preview behavior to all large text files (uniform UX) or only to those past the existing 5 MB cap.
• How to surface this in the file tree: the existing size column already shows the bytes; maybe a tooltip on the File tab "(preview only)" when in head-mode.

Out of scope

• Tail preview / random access — not needed for the typical "is this output sane?" use case.
• Pagination — same.
• Binary preview — separate concern.

Files touched

• `app/src/main/files-handler.ts` — add a streaming head-read path for files > MAX_READ_BYTES.
• `app/src/renderer/files/file-viewer.ts` — banner + read-only mode for preview content.
• IPC contract: `files:read` could grow a `{ truncated: true, headLines, lineLimit }` shape, or add a new `files:read-head` invoke.

Why now

Filed after a tester loaded a real Galaxy output into the analysis directory, opened the File tab, and got a useless error. Common enough that the current error-only behavior is a regression vs. just having no File tab at all.