[FEATURE] macOS desktop app: change font size without scaling the whole UI

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

On the Claude macOS desktop app, there is no way to increase reading text size independently of UI chrome. The only levers available (Cmd+ / Cmd- zoom, or macOS-wide Display scaling) scale everything — buttons, padding, sidebars, input area, message bubbles — not just the text. The result is that to get comfortable reading size, the whole interface balloons and horizontal real estate is lost, which on a laptop screen makes the app harder to use, not easier.

This is especially painful during long reading sessions (reviewing a model's long reasoning or a code block) where the natural move is "bump up the text a notch" — but doing so costs window layout.

Related but distinct: #48237 asks for a font-size setting for the Code tab on Windows. That's adjacent, but the core ask here is the decoupling itself (text size vs. UI scale) on macOS, not a per-tab control.

Proposed Solution

Add a text-size control in Settings > Appearance that adjusts only typographic size — body copy, code blocks, message content — while leaving UI chrome (toolbar, sidebar, input, spacing, icon sizes) at its native scale.

Ideal shape:

1. A "Text size" slider or stepped control (e.g. 90% / 100% / 110% / 125% / 150%) separate from any zoom shortcut.
2. Applies to all tabs (Chat, Code, Cowork) consistently.
3. Optional: a keyboard shortcut bound to text-only size (e.g. Cmd+Shift+= / Cmd+Shift+-) so the regular Cmd+= / Cmd+- can remain standard window zoom if desired, or be remapped.
4. Persisted per-user.

Alternative Solutions

• Cmd+ / Cmd- inside the app: scales the whole UI, defeats the purpose.
• macOS Display > Resolution: system-wide, affects every app, not practical.
• Accessibility > Zoom: magnifier-style, not a daily-use solution.
• Larger external monitor: doesn't help on the go.

None of these let the user do the one obvious thing: make text a bit bigger without rearranging the room.

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

1. I'm on a 14" MacBook reading a long response from the model.
2. Default text feels a touch small for sustained reading; I'd like ~115%.
3. I press Cmd+= — the whole UI scales up, the input box grows, the sidebar takes more room, fewer characters fit per line, the code block now wraps awkwardly.
4. I press Cmd+- back to default and squint instead.
5. With a text-only size control, I'd set text to 115% once, keep the layout I already like, and stop fighting the trade-off.

Additional Context

• Platform: macOS desktop app.
• Precedent: most reading apps (Kindle, Apple Books, Safari Reader, iA Writer) separate text size from window/UI scale. Electron apps can do the same by scoping a CSS font-size variable on a content root instead of using webFrame.setZoomFactor.
• Accessibility angle: users who want larger text for readability without sacrificing information density get a real path today they don't have.