fix(forms): show specific per-field validation errors instead of generic message

[iitzIrFan]

fixes: #5403

---

The newsletter box and the contact/partner forms all showed the same vague
"Invalid form, please review the fields." no matter what was actually wrong.

This adds a small shared util (src/utils/formValidation.ts) that turns the
browser's native ValidityState into proper messages, so now:

• the footer newsletter says "Please enter your email address." /
  "Please enter a valid email address."
• the contact, partner, and preview-access forms show an inline error under
  each invalid field (with a red border), focus the first one, and clear the
  error as you type
• the required consent checkbox on the partner form gets its own message too

---

[github-actions]

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🔦 Lighthouse Benchmark

Tested: http://localhost:8787 on 2026-09-02 10:54 UTC
Compared against main baseline from 2026-09-02

Scores (0–100, higher is better)

Page	Performance	Accessibility	Best Practices	SEO
Home	76	84	73 🟢 +19	92
Get Started	61	83	73 🟢 +19	100
Pricing	98	92	73 🟢 +19	100
Enterprise	78 🔻 -19	83	73 🟢 +19	100
Cloud	92	80	73 🟢 +19	100
About Us	83 🟢 +83	91	73 🟢 +19	100
Docs Landing	92	88	73 🟢 +19	92
Contribute to Kestra (simple docs)	98	88	54	92
Flow (full featured docs)	94	90	73 🟢 +19	92
Blog Index	65	91	73 🟢 +19	100
Blog Post (sample)	88 🔻 -10	88	73 🟢 +19	100
VS Page (sample)	98	88	73 🟢 +19	100
Plugins Landing	93	81	73 🟢 +19	92
Plugin Page (sample)	96	88	73 🟢 +19	100
Plugin Debug Page (sample)	97	88	73 🟢 +19	100
Plugin Debug Return Page (sample)	94	88	77 🟢 +19	100
Blueprints Landing	80	91	73 🟢 +19	100
Blueprint Audit Logs CSV Export	0	0	0	0

Core Web Vitals (lower is better)

Page	LCP	FCP	TBT	CLS	Speed Index
Home	1.29 s	0.67 s	421 ms	0.002	1.66 s
Get Started	2.06 s	0.64 s	971 ms 🔻	0.000	1.00 s
Pricing	1.04 s	0.58 s	29 ms	0.000	0.77 s
Enterprise	1.24 s	0.59 s	31 ms	0.424 🔻	0.70 s
Cloud	1.85 s	0.62 s	50 ms	0.001	0.88 s
About Us	2.96 s	0.67 s	28 ms	0.000	0.77 s
Docs Landing	1.61 s 🔻	0.53 s	123 ms	0.000	0.99 s
Contribute to Kestra (simple docs)	1.00 s	0.56 s	60 ms	0.002 🟢	0.73 s
Flow (full featured docs)	1.24 s 🟢	0.63 s	130 ms 🔻	0.000	1.13 s
Blog Index	19.62 s	0.56 s	21 ms	0.001	18.91 s
Blog Post (sample)	2.34 s 🔻	0.61 s	29 ms	0.000	0.78 s
VS Page (sample)	1.01 s	0.58 s	32 ms	0.064	0.59 s
Plugins Landing	1.02 s	0.61 s	27 ms	0.000	2.53 s
Plugin Page (sample)	0.94 s	0.64 s	21 ms 🟢	0.000	1.76 s 🟢
Plugin Debug Page (sample)	0.86 s	0.53 s	57 ms	0.000	1.70 s 🟢
Plugin Debug Return Page (sample)	0.92 s	0.62 s	85 ms 🔻	0.000	2.08 s
Blueprints Landing	2.35 s	0.94 s	33 ms	0.005	2.91 s
Blueprint Audit Logs CSV Export	0.00 s	0.00 s	0 ms	0.000	0.00 s

Legend

🟢 improved  ·  🔻 regressed  ·  (blank) no significant change
Score threshold: ±10 pts  ·  Metric threshold: ±30% of baseline

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