Add /resume page, link from nav and hero

[dmserrano]

What

• /resume: new page from src/content/pages/resume.md, rendered by the existing [...slug].astro + PageLayout. It's markdown, so edits are content changes.
• Nav: adds a "Resume" item after "Now".
• Hero: "Resume" is the first button in the link row, before GitHub/LinkedIn/Email/RSS. It's kept out of siteConfig.socials on purpose so it doesn't also show up in the footer.
• Icon: new file icon for the hero button.

Mobile nav fix (included because this PR caused most of it)

A fifth nav link pushed the header off-screen on phones. Measured at 375px: the four-item nav on main was already 19px over the screen edge, and adding Resume made it 91px. Mobile browsers respond by zooming out the whole page.

Below 520px, the nav links now go on their own full-width row under the brand. --nav-h goes up to 96px at that width so scroll-padding-top and the heading scroll-margin-top still clear the sticky header.

Width	Before	After
1280px	one row, 64px	unchanged
375px	91px overflow	no overflow; brand / links on 2 rows
360px	overflow	no overflow
320px	overflow	no overflow; links wrap to a 3rd row

Content notes

• Taken from the PDF (Desktop/Jobs/Dominic_Serrano_Resume.pdf, Sep 7) rather than career-coach/resume/*.md. The PDF is newer, with a different summary sentence and Experience before Projects.
• Phone number left out on purpose. Email, LinkedIn and GitHub are listed.
• Fixed a missing space in the PDF summary ("behind them.Recently").
• No PDF download yet, since the current PDF includes the phone number.

Checks

• astro build: 7 pages, /resume/ included
• vitest: 15/15 passing
• Measured the layout at 1280 / 375 / 360 / 320px (above)
• Eyeball it yourself in light and dark (the Browser pane was hidden, so no screenshots)