fix(deps): resolve all 10 open Dependabot security alerts

[Sagargupta16]

Fixes all 10 open Dependabot security alerts. Direct dep astro bumped in package.json; the six transitive packages are forced to patched versions via overrides in pnpm-workspace.yaml.

Alerts resolved

Alert	Package	Severity	Vulnerable range	First patched	Now locked
#11	nanoid	high	< 3.3.18	3.3.18	3.3.18
#9	nanoid	high	< 3.3.16	3.3.16	3.3.18
#10	postcss	medium	<= 8.5.22	8.5.23	8.5.26
#5	postcss	high	<= 8.5.17	8.5.18	8.5.26
#7	js-yaml	high	>= 4.0.0, < 4.3.1	4.3.1	4.3.2
#6	fast-uri	high	>= 3.0.0, < 3.1.5	3.1.5	3.1.6
#4	fast-uri	high	>= 3.0.0, <= 3.1.3	3.1.4	3.1.6
#3	fast-xml-parser	high	>= 5.9.3, < 5.10.1	5.10.1	5.11.1
#2	svgo	high	>= 4.0.0, < 4.0.2	4.0.2	4.1.0
#1	astro	medium	>= 2.9.0, <= 7.0.9	7.1.0	7.2.10

Changes

• package.json: astro ^7.0.7 -> ^7.1.0 (resolves to 7.2.10); version 0.2.0 -> 0.2.1
• pnpm-workspace.yaml: added overrides for nanoid@3, postcss@8, js-yaml@4, fast-uri@3, fast-xml-parser@5, svgo@4 (each scoped to the single major present in the tree)
• pnpm-lock.yaml: re-resolved
• CHANGELOG.md: 0.2.1 entry

Verification

• pnpm install: clean
• pnpm audit: no known vulnerabilities found
• pnpm build: pass (2 pages built)
• pnpm check: 0 errors, 0 warnings
• pnpm verify:content: pass
• pnpm exec prettier --check on touched files: pass
• pnpm test:e2e: 30 passed, 12 skipped (reader tests self-activate once chapters exist)