ui: UI/chat form follow ups (#26743)

* ui: split the markdown rendering setting per surface

User content and thinking get their own toggle again, so turning off
markdown for a message leaves reasoning blocks formatted. Both default
to markdown. A stored renderContentAsRawText unfolds onto the user key
and is dropped from the config.

File mentions render as badges in the raw text path too, through a
narrow pass over [name](file://path) that leaves everything else
untouched.

* ui: let the rich chat input scroll past its max height

The contenteditable renderer caps its height with max-height but had no
overflow rule, so a long buffer overflowed into the input area wrapper
and got clipped by its overflow-hidden, leaving no way to reach the
bottom of the message. The textarea renderer scrolls natively and was
never affected.

* ui: apply the new lint and format config

* ui: move the render keys unfolding into the migration service

Address review from @allozaur: the settings store no longer rewrites
persisted config on load, the raw text toggle now unfolds onto the
per-surface render keys in migration.service.ts, next to the other
config migrations. The mention scanner flag and the directory path
suffix become named constants.

tools/ui/tests/unit/mention-segments.test.ts

@@ -0,0 +1,48 @@
+import { splitMentionSegments } from '$lib/utils/mention-badge';
+import { describe, expect, it } from 'vitest';
+
+describe('splitMentionSegments', () => {
+	it('returns a single plain run when there is no mention', () => {
+		const segments = splitMentionSegments('# not a heading here');
+
+		expect(segments).toEqual([{ mention: null, text: '# not a heading here' }]);
+	});
+
+	it('splits text around a mention', () => {
+		const segments = splitMentionSegments('look at [main.c](file:///src/main.c) please');
+
+		expect(segments.map((segment) => segment.text)).toEqual([
+			'look at ',
+			'[main.c](file:///src/main.c)',
+			' please'
+		]);
+		expect(segments[1].mention).toEqual({ name: 'main.c', path: '/src/main.c' });
+	});
+
+	it('decodes percent-encoded paths', () => {
+		const segments = splitMentionSegments('[a b.txt](file:///tmp/a%20b.txt)');
+
+		expect(segments[0].mention?.path).toBe('/tmp/a b.txt');
+	});
+
+	it('keeps the directory marker so the folder icon is picked', () => {
+		const segments = splitMentionSegments('[src](file:///repo/src/)');
+
+		expect(segments[0].mention?.path).toBe('/repo/src/');
+	});
+
+	it('handles adjacent mentions with no text between them', () => {
+		const segments = splitMentionSegments('[a](file:///a)[b](file:///b)');
+
+		expect(segments).toHaveLength(2);
+		expect(segments.every((segment) => segment.mention !== null)).toBe(true);
+	});
+
+	it('preserves the exact source when segments are joined back', () => {
+		const source = 'see [a](file:///a) and [b](file:///b/) done';
+
+		expect(splitMentionSegments(source).reduce((acc, segment) => acc + segment.text, '')).toBe(
+			source
+		);
+	});
+});